This invention relates generally to the field of threaded closures for containers, and more particularly to synthetic resinous closures of relatively small diameter employed to seal containers of pressurized contents, typically soft drinks, beer and other carbonated comestibles. Traditionally, such containers have been sealed using metallic caps commonly referred to as crown closures requiring the use of a tool for removal. Over the past fifteen to twenty years, such closures have been almost completely replaced by a synthetic resinous closure either injection molded, or, more commonly, compression molded. Such closures are usually provided with a tamper-evident ring which depends from the lower edge of the cylindrical side wall of the closure, and is interconnected to said edge by frangible bridges formed during the molding operation, or by a separate slitting operation. Annual production of said closures surpasses that of other types of closures, and, not surprisingly, the art relating to such closures is in a highly developed state.
Although synthetic resinous closures of this type have many advantages, including the ability to be removed without the use of a tool, and the ability to reseal the container where all of the contents are not consumed upon first opening, they do present significant problems during manufacture.
One of the problems concerns the formation of a sealing liner beneath the inner surface of the end wall of the closure. The material used in forming the closure, while possessing a degree of resilience, is normally not possessed of adequate cold flow properties to form a gas tight seal against the mouth of the container neck or finish. This function is normally provided by forming a liner of softer synthetic material in situ using the closure as a female part of the mold in conjunction with a reciprocating male part which flattens and shapes a measured quantity of material. The closure is provided with an inwardly projecting lip which is of lesser diameter than the internal thread diameter forming an undercut or recess which shapes the rim of the flattened seal. That portion of the rim of the seal disposed immediately inwardly of the lip engages the end surface of the mouth of the finish to effect a gas tight seal. It is usual to form that portion to a slightly thicker cross section for greater resiliency. This construction does not permit the formation of a sealing effect which extends outwardly beyond the inner edge of the lip.
A more serious problem lies in the provision of frangible ribs or bridges which interconnect the lower edge of the side wall or skirt of the closure with the tamper-indicating band or ring. The bridges must be sufficiently strong so as to avoid breaking when the closures are installed, either by threading or pressing the closures into position, and yet be capable of readily fracturing when the closure is unthreaded. To this end, designs have included the provision of some bridge members which are stronger than other bridge members and are positioned in specific locations, this construction being partially successful when the tamper-evident ring is split and remains attached to the skirt during removal of the closure. Other constructions provide for a camming action utilizing inclined surfaces at the lower edge of the skirt and the upper edge of the tamper-indicating ring.
The most common construction used for fracturing the bridges is the use of flexible wings or tabs. One form uses tabs which engage the outer surface of the finish to prevent rotation of the ring relative to the skirt. More commonly, the tabs are pivoted about an axis normal to the axis of the closure and have free ends which engage beneath a radially projecting bead on the outer surface of the container finish. The tabs are not only pivoted relative to the inner surface of the ring, but are flexible as well to permit easy removal without damage from a forming mold. The relatively thin cross section required to produce this flexibility has an adverse result in that the tabs are occasionally not sufficiently rigid to assure prompt breakage of the bridges when the closure is unthreaded. This problem is accommodated by the provision of a radially inwardly extending band on the inner surface of the ring against which the tabs are engaged after being folded through approximately 135 degrees. This engagement reduces the effective diameter of the bead on the tamper-indicating ring to less than that of the corresponding bead on the outer surface of the container, so that it cannot pass the bead on the container, and continued unthreading of the closure serves to break the bridges. The use of this construction is almost mandatory where the bridges are of unequal strength. However, this construction in its so-called second mode of operation requires a substantial degree of unthreading of the closure before the bridges are broken, and no substantial leverage is obtained.